1. Field of the Invention
The present invention relates to LEDs and in particular to an improved line monitor for indium rich (green) InGaN LEDs.
2. Related Art
The rapidly emerging LED industry is poised to soon intersect the commercial and residential lighting market. As a result, growth in the LED industry is expected to accelerate dramatically. This emerging industry is also poised for the insertion of yield management technologies, just as silicon manufacturing was 30 years ago.
One of the most difficult problems in LED manufacturing is the growth of LEDs that emit light in the green region of the electronmagnetic spectrum. In general, LEDs are manufactured from indium gallium nitride (InxGa(1-x)N)(wherein X varies as indicated below and is deleted for simplicity hereafter). Indium compositions of only a few percent are required for “blue” LEDs. However, indium content from ten to twenty percent is required for bandgaps that provide light in the green region of the electromagnetic spectrum. As used herein, this content percentage of indium is referenced as “indium rich” InGaN.
Unfortunately, it is well known that indium rich InGaN quantum wells do not grow with uniform film composition. Due to the difference in surface mobilities, strain between the indium rich InGaN quantum wells and its corresponding GaN confinement layers, and other physical considerations, the indium rich InGaN material generally exhibits “clustering” of the indium. Namely, the quantum well is no longer homogeneous in indium composition but instead exhibits a film of widely varying indium composition.
FIG. 1 illustrates an exemplary cross-section of an LED 100 having an InGaN well. In LED 100, a substrate 101 is typically formed from sapphire or silicon carbide. Substrates of AlN or bulk GaN are also being considered. Barrier layer 102 is formed on substrate 101. N-type layer 103 is formed on barrier layer 102. P-type layer 105 is formed on n-type layer 103. N-type layer 103 and p-type layer 105 can be made from gallium nitride (GaN) with n-type and p-type doping, respectively. A quantum well 104, which is formed between n-type layer 103 and p-type layer 105, is actually a very thin well on the order of 10-20 nanometers thick, but is shown thicker for illustration purposes (i.e. compared to n-type layer 103 and p-type layer 105, which are both on the order of 0.5-1.0 micron thick). Quantum well 104 is formed from undoped indium gallium nitride. As shown in FIG. 2, which illustrates a plane view of quantum well 104, indium clusters 201, localized regions of relatively high In content, are not distributed uniformly in quantum well 104. Individual indium clusters are generally formed with characteristic diameters of the order of 10 nm each.
Numerous studies have been reported to understand the detailed nature of the clustering and its effect on device performance. It is generally agreed that clustering severely affects device performance with the result that “green” LEDs are 3-5 times less efficient than “blue” or “red” LEDs (wherein efficiency can be measured by Lumens Out/Watts In). This effect is called the “green gap” in the LED industry. This severe reduction in internal quantum efficiency for green devices severely constrains white light LED brightness, color balance, and thermal packaging considerations for all applications in which three primary color LEDs (i.e. green, blue, and red) are used to generate “white light”.
Currently, photoluminescence, electroluminescence, and transmission electron microscopy can be used to monitor post-epitaxial growth (i.e. a thin film of single crystal material formed over a single crystal substrate). Photoluminescence is typically performed with a tool that paints a spot size of roughly 5 microns diameter on the wafer. The use of a low NA (numerical aperture) lens and the wavelength of the light source yield this spot size. The result is that the average photo luminescence performance over a relatively large area of the wafer can be probed. Unfortunately, this area is roughly 2.5 to 3 orders of magnitude larger than the size of individual indium clusters 201. As a result, the fingerprints (i.e. any properties that can be measured and characterized) of indium clusters 201, which can indicate processed device performance, cannot be obtained using photoluminescence.
Electroluminescence probes even large areas of the wafer of course and therefore has even greater disadvantages. In contrast, transmission electron microscopy can be used to probe device morphology at high resolution. However, electron microscopy is very slow, expensive, and does not provide information on the manner in which morphology affects carrier mobility and recombination in quantum well 104 (such information being highly representative of LED performance).
Therefore, a need arises for an improved monitor for high-level indium InGaN quantum wells in LEDs.